This invention relates to a fuel injection apparatus for internal combustion engines, particularly diesel engines.
Several fuel injection apparatus have so far been proposed, e.g., by Japanese Patent Publication No. 47-38324 and Japanese Provisional Patent Publication No. 50-45124, which each use a servo piston having a large size portion defining a servo-pressure chamber which is intermittently supplied with pressurized fuel used as pressure operating fluid from a pressure feed source by way of a solenoid controlled selector valve to cause reciprocating motion of the servo piston. The reciprocating motion of the servo piston in turn causes compression of fuel in the pump working chamber which has been introduced into the chamber, directly from the above pressure feed source or by way of the above solenoid controlled selector valve, to force the fuel to be injected through an injection nozzle into an engine cylinder.
According to such conventional fuel injection apparatus, fuel, which is to be supplied to the engine, is also used as pressure operating fluid and supplied to the above servo-pressure chamber for causing reciprocating motion of the servo piston. That is, the fuel feed system is used as an operating fluid feed system for the servo piston, too. Fuel generally used in engines is relatively low in viscosity. However, to use such fuel in a hydraulic driving system for the servo piston including the operating fluid feed system under high pressure (e.g., 750 kg/cm.sup.2 or more), high lubricativeness and high fluidtightness are required of hydraulic devices used in such hydraulic driving system. If the hydraulic devices are not satisfactory in lubricativeness and fluidtightness, they cannot endure long use. Therefore, to have sufficient durability, the hydraulic devices have to meet special requirements in respect of lubricativeness and fluidtightness, which involves a problem of manufacturing cost.
Further, according to the above-mentioned conventional fuel injection apparatus, the fuel injection end is determined by the extreme compression stroke end point of the servo piston and cannot be electrically controlled. Therefore, if the fuel being supplied to the servo piston is subject to pressure variation, the injection quantity, the injection period, the injection timing, etc. all have to be controlled solely by changing the period and timing of energization and deenergization of the solenoid controlled selector valve. However, in fact setting of the timing of supply of an energization control signal to the selector valve is very difficult, and accordingly an electronic control circuit, which is usually used for control of supply of such control signal, is necessarily complicated in construction or circuit configuration, which is disadvantageous in respect of manufacturing cost and maintenance.